Edit: Just to add, this is to go into an application that will be running 24x7x365, with around 0.05A current draw for 99% of the time.
I'm wanting a cheap, safe and reliable way of taking AC power into my project box to power a 5V 0.4A (peak current) circuit. After looking at all the options it would seem the best way would be for me to have the following:
Isolation transformer -> Rectifier -> Smoothing cap -> Buck Switching Regulator
Which is certainly within the realms of possible. Although I've got a couple of questions:
1) If a transformer outputs 18VAC at 178mA (supplying power of 3.2VA), would the regulator be able to source 0.64A from it (5V / 3.2VA)... Obviously this is ignoring the voltage drops over the other components.
2) There is a very large price difference between 4 power diodes and a pre-made bridge rectifier, does a pre-made bridge rectifier have any benefit beyond the 4 diodes?
3) I don't need to go down this route because I'm requiring isolation for safety (hence the transformer), and in fairness I'm rather ignorant about flybacks etc. but could I not get an AC/DC converter (from this category on Farnell), which would replace the isolation transformer and rectifier then continue the schematic as above?